1. Field of the Invention
The present invention relates to a manufacturing method and a molding die for a resin part of a vehicular lamp. More particularly, the invention relates to such a manufacturing method and a molding die in which the time required for manufacturing and the cost of manufacturing a resin part are reduced while producing a part of improved smoothness.
2. Description of the Related Art
The resin-molded parts of a vehicular lamp generally include a reflective surface formed in at least one portion, such as a reflector of a headlamp or marker lamp, and an extension portion of a reflector. The reflective surface is generally formed by applying to the resin surface a vapor deposition coating of a metal such as aluminum.
A metallic vapor deposition coating closely follows the condition of the underlying resin surface, which serves as a substrate. Therefore, there is a problem that a smooth-appearing reflective surface, i.e., a reflective surface with a highly regular reflectance over its entire extent, cannot be obtained unless the surface of the underlying surface to which the metallic vapor deposition coating is applied has a mirror-like finish. Thus, conventionally an undercoating for the metallic vapor deposition coating is first applied to the surface of the injection-molded resin part which is to serve as a reflective surface, and the metallic vapor deposition coating is then formed on the undercoating layer.
The reasons why the undercoating layer is required will now be explained.
A cavity surface of a molding die for injection-molding a resin part, particularly a portion of the cavity surface corresponding to a surface which is to serve as a reflective surface, is finished to be mirror-like. Therefore, if the molded resin part directly follows the surface of the die, a high-quality mirror-like surface can be obtained. In general, however, there is a problem that an ideal mirror-like surface is difficult to obtain in injection molding.
The temperature of the die is approximately 40° C. to 100° C. while the temperature of molten resin is 200° C. or higher under general molding conditions. Therefore, at the instant when the injected molten resin comes into contact with the surface of the die (the cavity surface), it is cooled rapidly and a solidified layer is formed in the vicinity of the contact interface of the injected molten resin with the cavity surface. It is thought that this solidified layer reduces the transfer property for copying the cavity surface accurately.
In addition, an additive is added to the above-mentioned resin part for various reasons. When rapid cooling occurs in the vicinity of the contact interface of the molten resin with the cavity surface as described above, microscopic dents and bumps appear on the surface of the molten resin due to the difference between the shrinkage rates of the additive and the base polymer. These dents and bumps reduce the smoothness of the surface of the resin part and prevent it from being mirror-like.
Therefore, as described above, before the metallic disposition coating is applied, an undercoating layer is formed so as to increase the smoothness of the surface which is to serve as a substrate for vapor deposition.
However, since the step of forming the undercoating is interposed between the step of injection-molding the resin part and the step of forming the metallic vapor deposition coating, the number of the steps is increased. This results in an increase in cost.